This invention relates to a method for manufacturing an information memory disk on which information is to be recorded or has previously been recorded in an optical manner.
Conventional disks of this type are provided with a reflecting layer formed thereon, on which circular tracks are formed. In recording, a laser beam is converged on the track to form ruggedness signals thereon, or part of the reflecting layer is removed to form pit signals. The use of such optical disks in the recording enables or facilitates high-density recording, non-contacting reproduction, high-speed random access, prolonged stroage, simple recording system, additional recording, low cost per bit, etc. Thus, the optical disks may widely be used as media for both recording and reproduction or for reproduction use only. The disks for reproduction use only can be used as video disks for household use, video disks for commercial use, audio PCM disks, etc. As for the disks of the recording-reproduction type, they can be applied to document file systems, video file systems, universal memories for computers, etc.
One such prior art disk is generally composed of a transparent base of synthetic resin having a circular hole in its center, and a metallic reflecting layer attached to one surface of the base. The track on which information is to be recorded or has previously been recorded is in the form of a spiral groove or multi-ring grooves.
Pressing and injection molding are typical conventional methods for manufacturing the base. The pressing method is used in the manufacture of ordinary records or disks. In this method, a material is pressed between a lower die and an upper die bearing a stamper thereon. At the same time, the circular center hole is formed by means of a columnar projection at the central portion of one of the two dies. After the pressing is completed, the outer peripheral portion of a blank is shaved to determine the outside diameter thereof, and thus the base is finished. In the injection molding method, a resin material is injected between upper and lower dies to form a disk member, and then a circular hole is bored through the central portion of the disk member to form the base.
In the case of the pressing method, the accuracy of alignment between the center of the circular hole of the base and the center of the tracks depends on the accuracy of attachment of the stamper to the upper die. In this method, it is quite difficult to restrict the eccentricity to such a low degree that information can be recorded and reproduced at high density with a track width of 0.8 .mu.m to 0.9 .mu.m and a track pitch of 2 .mu.m or less. Thus, the track width and pitch cannot help being large.
In the latter case where the circular hole is formed after the injection molding, the outer periphery of the disk member is already fixed when the disk member is prepared. If the circular hole is formed in the basis of the outer periphery, therefore, the center of the circular hole will not meet the center of the previously formed track, resulting in the same difficulty as in the case of the pressing method. If the circular hole is formed in the basis of the track, on the other hand, the circular hole will be deviated as against the outside diameter of the base, possibly causing vibration.